Technical Field
This invention relates to magnetic gradiometer elements and magnetic gradiometers to detect living body magnetism of an ultra-minute magnetic field with a sharp magnetic gradient.
Background Art
SQUID equipment for detecting living body magnetism is widely used as diagnosis equipment such as magneto cardiogram and magnetoencephalography. Though SQUID has an ultra-high sensitivity in magnetic field detection, it needs a cryogenic temperature retainer and a magnetic shield room. It means that it is too expensive to spread this diagnostic technology. If a handy-type equipment to detect living body magnetism is developed, it will make popular diagnosis equipment.
However, the SQUID equipment has some problems besides being expensive. The first problem is that a magnetic field detection part of SQUID cannot be placed close to a surface of a living body and it can be placed at most at the distance of about 50 mm from the body, because it is contained in a cryogenic temperature retainer. The second problem is that its diameter is too big to make a fine pitch array arrangement and it cannot display fine images with very small pixels for magneto cardiogram or magnetoencephalography. The third problem is that it is difficult to detect living body magnetism three-dimensionally by arranging a magnetic field detection part along perpendicular direction of Z-axis against the horizontal direction formed by X-axis and Y-axis. The last one is that it is difficult to specify the depth position of the signal source because plural magnetic field parts cannot be arranged in the direction of Z-axis.
To solve the above weak points of SQUID equipment for detecting living body magnetism, a lot of efforts have been made to develop an ultra-sensitive micro magnetic sensor that detects living body magnetism using FG sensor, MI sensor, GSR sensor based on GHz-Spin-Rotation effect, and tMR sensor. FG sensor and MI sensor can detect living body magnetism of 5 pT-level using mechanical coil, but its size and price as well as the detectability is not sufficient. MI sensor and tMR sensor using micro elements produced by photolithography method have advantage in size and cost, but the detectability to detect living body magnetic fields is poor beyond 100 pT level at this moment.
A differential type of element or a gradiometer-type element is considered to be effective in theory to omit the magnetic shield room but still presents difficult technical problems. Nowadays many research projects have been carried out with trial and error. Patent Document 1 shows a biomedical signal detecting equipment in which a magnetic head has a magnetism detecting part with 1-cm amorphous wire and a pulse generator or high frequency generator. The detecting level of biomagnetism signal is 100 pT.
Patent Document 2 shows a magnetic coupling type gradiometer to measure the gradient of magnetic field strength. The distance between its detecting part and the object to be detected along the amorphous wire is about 150 mm and living body magnetism is not its target.
Non-Patent Document 1 shows a differential-type MI element in which two MI elements are arranged by mechanical ways along a reference line to detect the difference in magnetic field strength between both elements. It achieves a detectability of 10 pT-level which is not sufficient for measuring living body magnetism. Because the mechanical assembly cannot make arrangement with no displacement mismatch between both elements and remains several microns slide against a base line.
There is a need to develop handy-type body equipment to detect living body magnetism which has detectability of magnetic field equivalent to that of the SQUID type equipment and is free from cryogenic temperature retainer and magnetic shield room as well as advantages which are high array density, 3-axis magnetic field detection and the depth position detection of the signal source. It is also a key to invent a magnetic gradiometer element of use in that.